Newbie Needin Help!

Hey. I have a 92 suburban, two 4x6 infinity's in the front and to ****** stock 4x10's in the back, powered by a kenwood deck( forgot model). Anyway I am looking to get a sub ans amp for my car. I was looking at the 12w3v2 JL's, the Jl website says they should run at about 300 watts, i liked them cuz they delivered nice bass, but were still clear. Now I need to decide on an amp, but first I need to know if I need to make any modifications, such as buy a new alternator. Please lend some help to me.